Polishing composition

ABSTRACT

The present invention relates to a polishing composition containing an aqueous medium and silica particles, wherein the silica particles in the polishing composition has a zeta potential of from −15 to 40 mV; a method for manufacturing a substrate including the step of polishing a substrate to be polished with a polishing composition containing an aqueous medium and silica particles, wherein the silica particles in the polishing composition has a zeta potential of from −15 to 40 mV; and a method for reducing scratches on a substrate to be polished with a polishing composition containing an aqueous medium and silica particles, including the step of adjusting a zeta potential of silica particles in the polishing composition to −15 to 40 mV. The polishing composition can be favorably used in polishing the substrate for precision parts, including substrates for magnetic recording media such as magnetic discs, optical discs and opto-magnetic discs; photomask substrates; optical lenses; optical mirrors; optical prisms; semiconductor substrates; and the like.

This application is a Divisional of co-pending application Ser. No. 11/081,560, filed on Mar. 17, 2005, the entire contents of which are hereby incorporated by reference and for which priority is claimed under 35 U.S.C. § 120.

FIELD OF THE INVENTION

The present invention relates to a polishing composition and a method for manufacturing a substrate.

In addition, the present invention relates to a method for increasing a polishing rate of a substrate (hereinafter referred to as a “polishing rate-increasing method”), a method for manufacturing a substrate using the method, a polishing composition, and a method for reducing scratches.

BACKGROUND OF THE INVENTION

Currently, steps for polishing various substrates have been employed in the manufacture of various kinds of substrates. For example, in the field of semiconductors, there has been employed a step of polishing a silicon wafer substrate; a compound semiconductor wafer substrate made of a compound such as gallium arsenide, indium phosphide, or gallium nitride; or a silicon oxide film, a metal film made of aluminum, copper, tungsten or the like, or a nitride film made of silicon nitride, silicon oxynitride, tantalum nitride, titanium nitride or the like, the film being further formed on the water. In the field of memory hard disks, there has been employed a step of polishing an aluminum substrate or glass substrate. In the field of display devices such as lenses and liquid crystals, there has been employed polishing of glass. In the polishing step for these substrates to be polished, the polishing rate is important in order to increase the productivity, and various techniques for increasing the polishing rates have been proposed.

In recent memory hard disk drives, high storage capacity and miniaturization have been demanded. In order to increase the recording density, it has been strongly urged to lower the flying height of a magnetic head and to reduce the unit recording area. Along with this trend, the surface qualities required after polishing have become severely assessed every year even in a method for manufacturing a substrate for a magnetic disk. In order to satisfy the lowering of flying height of the magnetic head, the surface roughness, the microwaviness, the roll-off and projections are required to be reduced, and in order to satisfy the reduction in unit recording area, the acceptable number of scratches per one side of the substrate have been reduced, and the sizes and depths of the scratches have become increasingly smaller.

Also, in the field of semiconductors, highly integrated circuits and higher speed at the operating frequencies have been advanced, and the production of thinner wiring is required especially in highly integrated circuits. As a result, in the method for manufacturing a substrate for semiconductors, since the focal depth becomes more shallow with the increase in resolution required for an exposure device during the exposure of a photoresist, even more improvement in surface smoothness and planarization is desired.

Conventionally, for these polishing applications, a slurry polishing liquid mainly containing silica particles or cerium oxide particles has been used. The slurry polishing liquid containing the silica particles has been highly useful and is widely used, but has a disadvantage that the polishing rate is low. On the other hand, the slurry polishing liquid containing cerium oxide particles has been used for polishing optical glass, a memory hard disk made of glass, a semiconductor insulation film, or the like, and has a feature of a high polishing rate, but has a disadvantage that scratches tend to be easily formed.

In view of these disadvantages, JP2002-97459 A discloses a polishing agent for simultaneously reducing scratches and dust, while increasing the polishing rate by providing a silicon oxide film to be polished, with an aqueous dispersion slurry liquid containing oxide particles of which constituting atom is cerium, wherein the zeta potential of the surface of the particles is controlled to −10 mV or less. However, although scratches and dust are reduced as compared to the case where the zeta potential of the surface of the particles exceeds −10 mV, the polishing rate is also lowered, so that both the reduction of scratches and dust and the increase in polishing rate cannot be satisfied.

Further, JP2001-329250 A discloses a cerium oxide polishing agent for polishing a surface to be polished, such as a SiO₂ insulation film, at a high speed without damage, from a slurry prepared by dispersing cerium oxide particles in a medium, wherein the zeta potential of the surface of the particles is controlled to −100 mV to −10 mV. However, the description of the polishing at a high speed is made on the basis of comparison with a polishing agent containing silica particles, which are heterogeneous particles, so that the relationship between the polishing rate and the zeta potential has not yet been elucidated.

Moreover, JP2003-193037 A discloses a polishing composition for improving surface smoothness of a memory hard disk substrate. However, the surface smoothness is still insufficient for obtaining surface smoothness required for high density of the memory hard disk substrate.

SUMMARY OF THE INVENTION

The present invention relates to the following:

-   [1] a polishing composition containing an aqueous medium and silica     particles, wherein the silica particles in the polishing composition     have a zeta potential of from −15 to 40 mV; -   [2] a method of polishing a glass substrate with the polishing     composition as defined in the above [1]; -   [3] a method of polishing a memory hard disk substrate with the     polishing composition as defined in the above [1]; -   [4] a method for manufacturing a substrate including the step of     polishing a substrate to be polished with a polishing composition     containing an aqueous medium and silica particles, wherein the     silica particles in the polishing composition has a zeta potential     of from −15 to 40 mV; -   [5] a method for increasing a polishing rate of a substrate to be     polished with a polishing composition containing an aqueous medium     and silica particles, including the step of adjusting a zeta     potential of silica particles in the polishing composition to −15 to     40 mV; -   [6] a method for manufacturing a substrate including the step of     applying the method as defined in the above [5] to a substrate to be     polished; and -   [7] a method for reducing scratches on a substrate to be polished     with a polishing composition containing an aqueous medium and silica     particles, including the step of adjusting a zeta potential of     silica particles in the polishing composition to −15 to 40 mV.

DETAILED DESCRIPTION OF THE INVENTION

As a result of intensive studies on the requirements for achieving surface smoothness required for high density and high integration of a substrate for a precision part such as a memory hard disk substrate or a semiconductor substrate, the present inventors have found for the first time that the generation of “nano scratches” (fine scratches on a substrate surface having a depth of 10 nm or more and less than 100 nm, a width of 5 nm or more and less than 500 nm, and a length of 100 μm or more) which could not be so far detected inhibit the high density in a memory hard disk substrate, and high integration in a semiconductor substrate. The present invention has been accomplished thereby.

Specifically, the present invention relates to a polishing composition being capable of giving a polished object small surface roughness and remarkably reduced nano scratches, and having a high polishing rate, and a method for manufacturing a substrate having small surface roughness and remarkably reduced nano scratches.

Also, the present invention relates to a method for increasing the polishing rate of a substrate to be polished, while satisfying together the improvement in the surface smoothness of a surface to be polished, and a method for manufacturing a substrate having excellent smoothness and high productivity using the above method.

By using the polishing composition of the present invention in, for example, a polishing step for a substrate for a precision part for high density and high integration, the polished substrate has excellent surface smoothness and is capable of remarkably reducing conventionally undetected fine nano scratches at a high polishing rate. Therefore, there is exhibited an effect that a high-quality memory hard disk substrate and a substrate for a precision part such as a semiconductor substrate, each having excellent surface properties, can be efficiently manufactured.

These and other advantages of the present invention will be apparent from the following description.

1. Polishing Composition

One of the features of the polishing composition of the present invention resides in that the polishing composition is a polishing composition containing an aqueous medium and silica particles, wherein the silica particles in the polishing composition have a zeta potential of from −15 to 40 mV. By polishing a substrate to be polished with the polishing composition, the polishing rate can be increased while also satisfying surface smoothness.

The present invention makes it possible to provide excellent surface properties and to remarkably reduce nano scratches causative of the surface defects by adjusting the zeta potential of the silica particles within a range of from −15 to 40 mV, preferably from −15 to 30 mV. The nano scratches are important properties in obtaining high density and high integration especially in the memory hard disk substrate or semiconductor substrate. Therefore, by using the polishing composition of the present invention, a high-quality memory hard disk substrate having excellent surface properties or a semiconductor substrate can be manufactured at a high polishing rate.

Although not wanting to be limited by theory, the mechanism for reducing the nano scratches has not been elucidated, and it is presumably as follows. The closer the zeta potential of the silica particles approximates an isoelectric point, the larger the intergranular attraction between the silica particles, so that the detachment of coarse grains or aggregates of fine particles which are considered to cause scratches in the polishing to a surface of a substrate to be polished is suppressed.

In the present invention, the zeta potential refers to a potential obtained from an electrophoresing rate of an abrasive when an electric field is applied to the silica particles in the polishing composition from external. The determination device for the zeta potential is preferably, for example, those devices using the principle of electrophoresis, such as “ELS-8000” (commercially available from Otsuka Electronics Co., Ltd.), “DELSA440SX” (commercially available from Beckmann Coulter, Inc.) and “NICOMP Model 380” (commercially available from Particle Sizing Systems). Also, the determination can be substituted by applying the principle of ultrasonic wave method such as “DT1200” (commercially available from NIHON RUFUTO Co., Ltd.). In the determination according to the principle of electrophoresis, it is necessary to dilute the concentration of the silica particles in principle of the device. The zeta potential of the silica particles in the polishing composition in the present specification refers to a zeta potential of a polishing composition of which silica particle concentration is adjusted to a given concentration by an aqueous solution for adjusting zeta potential, the aqueous solution of which pH is previously adjusted to be the same as that of the polishing composition (an aqueous solution composed of a zeta potential-adjusting agent of the polishing composition and water, provided that in a case where two or more kinds of zeta potential-adjusting agents are contained in the polishing composition, an aqueous solution is prepared by keeping the content ratio thereof). In addition, the supernatant by centrifugation of the polishing composition can be used in place of the above-mentioned aqueous solution for adjusting zeta potential. Also, when the zeta potential is determined with the above-mentioned zeta potential-determination device, the determinations are repeated at least three times with the same sample under the same determination conditions in order to increase the reliability of the found value, and an average of these values is defined as a zeta potential.

The polishing rate can be increased by adjusting the zeta potential of the silica particles of the polishing composition of the present invention within a range from −15 to 40 mV. It is desired that the zeta potential is adjusted within a range from −15 to 30 mV, preferably from −15 to 20 mV, more preferably from −15 to 10 mV, even more preferably from −10 to 10 mV, even more preferably from −5 to 5 mV, from the viewpoint of reducing nano scratches.

In addition, it is desired that the above-mentioned zeta potential is adjusted within a range from −15 to 30 mV, preferably from −10 to 30 mV, more preferably from −5 to 30 mV, from the viewpoint of reducing nano scratches.

Incidentally, in the present invention, the adjustment of the zeta potential of the polishing composition is not particularly limited, and it is preferable that the adjustment is carried out before polishing. In addition, it is preferable that the zeta potential is kept within the above-mentioned specified range until the polishing is terminated. A specific method for adjusting the zeta potential will be described later.

The silica particles in the present invention include colloidal silica particles, fumed silica particles, and the like. The colloidal silica can be obtained according to a water glass method using an alkali metal silicate such as sodium silicate as a raw material, subjecting the raw materials to a condensation reaction in an aqueous solution, and allowing the silica particles to grow, or according to an alkoxysilane method using tetraethoxysilane or the like as a raw material, subjecting the raw material to a condensation reaction in a water-soluble organic solvent-containing water, such as an alcohol, and allowing the silica particles to grow. The fumed silica can be obtained by a method using a volatile silicon-containing compound such as silicon tetrachloride as a raw material, and subjecting the raw material to a vapor phase hydrolysis under a high temperature of 1000° C. or more with an oxyhydrogen burner.

Further, as the silica particles in the present invention, surface-modified silica particles, composite silica particles and the like can be used. The surface-modified silica particles refer to those in which a metal such as aluminum, titanium or zirconium, or an oxide thereof is adsorbed and/or bound to the surface of the silica particles, directly or via a coupling agent, or those in which a silane coupling agent, a titanium coupling agent or the like is bound. The composite silica particles refer to those in which nonmetal particles, such as polymer particles, and silica particles are adsorbed and/or bound. These silica particles can be used alone or in admixture of two or more kinds. Among these silica particles, the colloidal silica is preferable from the viewpoint of reducing scratches.

The silica particles have an average primary particle size, regardless of whether or not one or more kinds of silica particles are used in admixture, of preferably 1 nm or more and less than 40 nm, more preferably from 1 to 35 nm, even more preferably from 3 to 30 nm, even more preferably from 5 to 25 nm, even more preferably from 5 to 20 nm, from the viewpoint of increasing the polishing rate as to the lower limit, and from the viewpoint of reducing surface roughness (an average surface roughness: Ra, a peak-to-valley value: Rmax) as to the upper limit. Further, when the primary particles are aggregated to form secondary particles, the secondary average particle size is preferably from 5 to 150 nm, more preferably from 5 to 100 nm, even more preferably from 5 to 80 nm, even more preferably from 5 to 50 nm, even more preferably from 5 to 30 nm, from the viewpoint of increasing the polishing rate as to the lower limit, and from the viewpoint of reducing surface roughness as to the upper limit in the same manner as above.

In addition, the silica particles have a particle size distribution, regardless of whether or not one or more kinds of silica particles are used in admixture, such that D90/D50 is preferably from 1 to 5, more preferably from 1 to 4, even more preferably from 1 to 3, from the viewpoint of achieving reduction of scratches, reduction of surface roughness and a high polishing rate.

Incidentally, the average primary particle size of the silica particles, the particle size at 50% counted from a smaller particle size side of the primary particles in a cumulative particle size distribution on the number basis (D50), and the particle size at 90% counted from a smaller particle size side of the primary particles in a cumulative particle size distribution on the number basis (D90) can be each determined, regardless of whether or not one or more kinds of silica particles are used in admixture, by the method described below. Specifically, the photographs of the silica particles observed by a transmission electron microscope “JEM-2000 FX” commercially available from JEOL LTD. (80 kV, magnification: 10000 to 50000) are incorporated into a personal computer as image data with a scanner connected thereto. The projected area diameter of each silica particle is determined using an analysis software “WinROOF” (commercially available from MITANI CORPORATION), and considered as the diameter of the silica particles. After analyzing data for 1000 or more silica particles, the volume of the silica particles are calculated from the diameters of the silica particles based on the analyzed data using a spreadsheet software “EXCEL” (commercially available from Microsoft Corporation). The average primary particle size and D50 as referred to herein mean the same thing.

The average secondary particle size of the silica particles refers to a particle size at 50% counted from a smaller particle size side of the particles in a cumulative particle size distribution on a volume basis, as determined by electrophoretic light scattering method, regardless of whether or not one or more kinds of silica particles are used in admixture. As the determination device for electrophoretic light scattering method, there can be preferably used, for example, “ELS-8000” (commercially available from Otsuka Electronics Co., Ltd.), “DELSA 440SX” (commercially available from Coulter Beckman, Inc.) and “NICOMP Model 380” (commercially available from Particle Sizing Systems).

The content of the silica particles is preferably from 1 to 50% by weight, more preferably from 2 to 40% by weight, even more preferably from 3 to 30% by weight, even more preferably from 5 to 25% by weight, of the above-mentioned polishing composition, from the viewpoint of increase in the polishing rate and improvement in surface qualities.

The aqueous medium in the present invention refers to water and/or a water-soluble organic solvent. Water includes ion exchange water, distilled water, ultrapure water and the like. The water-soluble organic solvent includes primary to tertiary alcohols, glycols and the like. The content of the aqueous medium corresponds to the balance after subtracting the contents of the silica particles, the zeta potential-adjusting agent, and other components added as occasion demands from the entire weight (100% by weight) of the polishing composition. The content of this medium is preferably from 60 to 99% by weight, more preferably from 70 to 98% by weight, even more preferably from 75 to 98% by weight, of the polishing composition.

The adjustment of the zeta potential of silica particles in the polishing composition can be effectively carried out by adding a zeta potential-adjusting agent to a polishing composition. The zeta potential-adjusting agent refers to an agent for controlling the surface potential of the silica particles by directly or indirectly adsorbing the agent to the surface of the silica particles or changing the property such as the degree of acidity or basicity of the medium of the polishing composition. The zeta potential-adjusting agent includes, for example, an acid, a base, a salt and a surfactant.

The zeta potential-adjusting agent is used, for example, as follows. When the zeta potential of the silica particle surface contained in the polishing composition exceeds 40 mV, as the zeta potential-adjusting agent, it is preferable to shift the zeta potential to a negative side with an acid, an acidic salt, or an anionic surfactant. On the other hand, when the zeta potential of the silica particle surface contained in the polishing composition is lower than −15 mV, as the zeta potential-adjusting agent, it is preferable to shift the zeta potential to a positive side with a base, a basic salt or a cationic surfactant. In addition, a neutral salt, a nonionic surfactant or an amphoteric surfactant may be used in the case where the zeta potential is adjusted without changing the pH of the polishing composition.

As the acid, an inorganic acid or organic acid may be used. The inorganic acid includes hydrochloric acid, nitric acid, sulfuric acid, phosphoric acid, a polyphosphoric acid, amide sulfuric acid, and the like. Also, the organic acid includes a carboxylic acid, an organic phosphonic acid, an amino acid and the like. The carboxylic acid includes, for example, a monocarboxylic acid such as acetic acid, glycolic acid, and ascorbic acid; a dicarboxylic acid such as oxalic acid and tartaric acid; a tricarboxylic acid such as citric acid. The organic phosphonic acid includes, for example, 2-aminoethylphosphonic acid, 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP), aminotri(methylenephosphonic acid), ethylenediaminetetra(methylenephosphonic acid), diethylenetriaminepenta(methylenephosphonic acid), and the like. In addition, the amino acid includes, for example, glycine, alanine and the like. Among them, the carboxylic acid and the organic phosphonic acid are preferable, from the viewpoint of reducing scratches and reducing nano scratches. For example, hydrochloric acid, nitric acid, sulfuric acid, phosphoric acid, a polyphosphoric acid, glycolic acid, oxalic acid, citric acid, 1-hydroxyethylidene-,1-diphosphonic acid, aminotri(methylenephosphonic acid), ethylenediaminetetra(methylenephosphonic acid), or diethylenetriaminepenta(methylenephosphonic acid) is suitably used.

The base includes an aqueous ammonia, hydroxylamine, an alkylhydroxylamine, a primary to tertiary alkylamine, an alkylenediamine, and alkylammonium hydroxide, and the like. The preferred base is an aqueous ammonia or an alkanolamine, from the viewpoint of reducing scratches and reducing nano scratches.

In addition, the salt includes salts of the above-mentioned acid. The cation for forming the salt is preferably those metals belonging to the Group 1A, 2A, 3B or 8 of the Periodic Table (long period form), ammonium, hydroxyammonium, an alkanolammonium or the like. Among them, the basic salt includes ammonium chloride, ammonium nitrate, ammonium sulfate, aluminum nitrate, aluminum sulfate, aluminum chloride, and the like. The basic salt includes sodium citrate, sodium oxalate, sodium tartrate and the like. The neutral salt includes sodium chloride, sodium sulfate, sodium nitrate, and the like.

The surfactant includes a low-molecular weight surfactant and a high-molecular weight surfactant, which is an agent that is adsorbed or chemically bound to the surface of the silica particles, and has one or more hydrophilic groups which may be identical or different in the molecule. Especially, the surfactant includes a nonionic surfactant having a nonionic group as represented by an ether group (an oxyethylene group or the like), or a hydroxyl group; an anionic surfactant having an anionic group, as represented by a carboxylate group, a sulfonate group, a sulfuric ester group or a phosphoric ester group; a cationic surfactant having a cationic group represented by a quaternary ammonium; and an amphoteric surfactant having an anionic group and a cationic group.

In addition, as the preferred combination of the above-mentioned silica particles and the zeta potential-adjusting agent, the zeta potential-adjusting agent is preferably hydrochloric acid, nitric acid, sulfuric acid, phosphoric acid, a polyphosphoric acid, glycolic acid, oxalic acid, citric acid, 1-hydroxyethylidene-1,1-diphosphonic acid, aminotri(methylenesulfonic acid), ethylenediaminetetra(methylenephosphonic acid), or diethylenetriaminepenta(methylenephosphonic acid), more preferably hydrochloric acid, nitric acid, sulfuric aid, phosphoric acid, citric acid, or 1-hydroxyethylidene-1,1-diphosphonic acid.

In addition, in the polishing composition used in the present invention, alumina particles can be used together, from the viewpoint of increasing the polishing rate. When the silica particles and the alumina particles are used together, the zeta potential-adjusting agent is preferably sulfuric acid, ammonium sulfate, phosphoric acid, a polyphosphoric acid, oxalic acid, citric acid, or 1-hydroxyethylidene-1,1-diphosphonic acid, more preferably sulfuric acid, ammonium sulfate, phosphoric acid, a polyphosphoric aid, citric acid, or 1-hydroxyethylidene-1,1-diphosphonic acid. Incidentally, it is preferable that the average primary or secondary particle size of the alumina particles is within the same range as those for the above-mentioned silica particles.

Here, the content of the zeta potential-adjusting agent in the polishing composition cannot absolutely be limited because the content is determined depending upon the property of the liquid of the polishing composition, the property of the silica particles, and the obtained zeta potential. For example, the content of the zeta potential-adjusting agent is preferably from 0.01 to 20% by weight, more preferably from 0.05 to 15% by weight, of the polishing composition, from the viewpoint of reducing scratches and reducing nano scratches. In addition, the zeta potential-adjusting agent may be previously contained in the polishing composition, or the zeta potential-adjusting agent may be contained in the polishing composition immediately before polishing.

Additionally, in the present invention, other abrasives can be used together with the silica particles. As the other abrasive, abrasives that are generally used for polishing can be used. The abrasive includes metals; carbides of metals or metalloids, nitrides of metals or metalloids, oxides of metals or metalloids or borides of metals or metalloids; diamond, and the like. The elements for metals or metalloids include those elements belonging to the Group 2A, 2B, 3A, 3B, 4A, 4B, 5A, 6A, 7A or 8 of the Periodic Table (long periodic form). Specific examples of the abrasives include aluminum oxide (hereinafter referred to as alumina), silicon carbide, diamond, magnesium oxide, zinc oxide, titanium oxide, cerium oxide, zirconium oxide, and the like; those in which the surface of these abrasives is subjected to modification or surface improvement with a function group; those formed into composite particles with the surfactant or the abrasive; and the like. It is preferable to use one or more kinds of these abrasives from the viewpoint of reducing surface roughness.

The abrasive has an average primary particle size, regardless of whether or not one or more kinds of the abrasives are used in admixture, of 1 nm or more and less than 40 nm. The abrasive has an average primary particle size of preferably 3 nm or more, more preferably 5 nm or more, from the viewpoint of increasing the polishing rate, and the abrasive has an average primary particle size of preferably 35 nm or less, more preferably 30 nm or less, even more preferably 25 nm or less, even more preferably 20 nm or less, from the viewpoint of reducing surface roughness (an average surface roughness: Ra, a peak-to-valley value: Rmax). Therefore, the average primary particle size is preferably from 1 to 35 nm, more preferably from 3 to 30 nm, even more preferably from 5 to 25 nm, even more preferably from 5 to 20 nm, from the viewpoint of economically reducing surface roughness. Further, when the primary particles are aggregated to form secondary particles, the secondary average particle size is preferably from 5 to 150 nm, more preferably from 5 to 100 nm, even more preferably from 5 to 80 nm, even more preferably from 5 to 50 nm, even more preferably from 5 to 30 nm, from the viewpoint of increasing the polishing rate, and from the viewpoint of reducing surface roughness of the substrate in the same manner as above.

Incidentally, the average primary particle size of the abrasive (except for the silica particles), regardless of whether or not one or more kinds of the abrasives are used in admixture, is determined by obtaining a particle size at 50% counted from a smaller particle size side of the primary particles in a cumulative particle size distribution on the number basis (D50) by using an image observed with a scanning electron microscope (magnification preferably from 3000 to 100000), and this D50 is defined as an average primary particle size. Here, one of the average primary particle size employs an arithmetic mean of breadth and length (an average of length and breadth). In addition, the secondary average particle size can be determined as a volume-average particle size using a laser diffraction method.

In addition, the abrasive (except for the silica particles) has a particle size distribution, regardless of whether or not one or more kinds of the abrasives are used in admixture, such that D90/D50 is preferably from 1 to 5, more preferably from 2 to 5, even more preferably from 3 to 5, from the viewpoint of achieving reduction of nano scratches, reduction of surface roughness and a high polishing rate. Here, D90 refers to a particle size at 90% counted from a smaller particle size side of the primary particles in a cumulative particle size distribution on the number basis (D90) by using an image observed with a scanning electron microscope (magnification preferably from 3000 to 100000).

The content of the abrasive (except for the silica particles) is preferably 0.5% by weight or more, more preferably 1% by weight or more, even more preferably 3% by weight or more, even more preferably 5% by weight or more, of the polishing composition, from the viewpoint of increasing the polishing rate. In addition, the content of the abrasive (except for the silica particles) is preferably 20% by weight or less, more preferably 15% by weight or less, even more preferably 13% by weight or less, even more preferably 10% by weight or less, of the polishing composition, from the viewpoint of improving surface qualities. Specifically, the content of the abrasive (except for the silica particles) is preferably from 0.5 to 20% by weight, more preferably from 1 to 15% by weight, even more preferably from 3 to 13% by weight, even more preferably from 5 to 10% by weight, of the polishing composition, from the viewpoint of economically improving surface qualities.

In addition, in the polishing composition usable in the present invention, other components can be formulated as occasion demands. The other components include an oxidizing agent such as hydrogen peroxide, a radical scavenger, a clathrate compound, an anticorrosive agent, a defoaming agent, an anti-bacterial agent and the like. The content of these other components is preferably from 0 to 10% by weight, more preferably from 0 to 5% by weight, of the polishing composition, from the viewpoint of polishing rate. The above-mentioned polishing composition can be prepared by properly mixing the above-mentioned components.

The concentration of each component in the above-mentioned polishing composition may be any concentration during the preparation of the composition and the concentration upon use. In many cases, the polishing composition is usually prepared as a concentrate, which is diluted upon use.

The pH of the above-mentioned polishing composition may be determined depending upon the silica particles used and the degree of surface modification such as surface treatment, from the viewpoint of polishing rate, reduction in scratches, and reduction in nano scratches. In the case where the silica particles are composed of colloidal silica, the pH is preferably 9 or less, more preferably 7 or less, even more preferably 6 or less, even more preferably 5 or less, even more preferably 4 or less, even more preferably 3 or less, even more preferably 2.5 or less, even more preferably 2 or less.

Since the polishing composition having the above constitution is used, there can be efficiently manufactured a polished substrate such as a substrate for precision parts having excellent surface properties such that there are very little scratches, especially nano scratches.

The nano scratches in the present invention refer to fine scratches on a substrate surface having a depth of 10 nm or more and less than 100 nm, a width of 5 nm or more and less than 500 nm, and a length of 100 μm or more. The nano scratches can be detected by an atomic force microscope (AFM), and can be quantitatively evaluated as the number of nano scratches as determined by “Micromax” a visual testing device as described in Examples set forth below.

In addition, the scratches refer to scratches on a substrate surface having a depth of 100 nm or more.

In addition, the evaluation method for surface roughness, which is a measure of surface smoothness, is not limited. In the present invention, the surface roughness is evaluated as roughness that can be detected as a short wavelength of 10 μm or less in the AFM (atomic force microscope), and expressed as an average surface roughness Ra. Specifically, the surface roughness is obtained according to the method described in Examples set forth below.

The polishing composition of the present invention can be used for polishing an object to be polished by, for example, feeding to a polishing device equipped with a jig having a substrate to be polished and a polishing cloth. By this process, the polishing rate of a substrate to be polished can be increased while at the same time satisfying surface smoothness of the substrate (reduction of scratches and nano scratches). The surface of a substrate to be polished is polished by pressing to a polishing device equipped with a jig having a substrate to be polished, or setting a substrate to be polished with polishing platens to which a polishing cloth made of a foamed article or non-foamed article made of an organic polymer or the like or a nonwoven fabric is attached; feeding the polishing composition to a surface to be polished; and moving the polishing platens or the substrate to be polished, while applying pressure.

The polishing load during polishing is preferably from 0.5 to 20 kPa, more preferably from 1 to 20 kPa, even more preferably from 3 to 20 kPa, from the viewpoint of an increase in the polishing rate and easy control of polishing.

The flow rate of the polishing composition to a substrate to be polished is preferably from 0.01 to 3 mL/minute, more preferably from 0.05 to 2.5 mL/minute, even more preferably from 0.1 to 2 mL/minute, per 1 cm² of the substrate, from the viewpoint of an increase in the polishing rate and easy control of nano scratches.

The material of a substrate to be polished, which is an object to be polished, with the polishing composition of the present invention includes, for example, metals or metalloids such as silicon, aluminum, nickel, tungsten, copper, tantalum and titanium, and alloys thereof; glassy substances such as glass, glassy carbon and amorphous carbons; ceramic materials such as alumina, silicon dioxide, silicon nitride, tantalum nitride, and titanium carbide; resins such as polyimide resins; and the like.

Among them, a substrate to be polished is preferably made of a metal such as aluminum, nickel, tungsten or copper, or made of an alloy containing these metals as the main components. For example, an Ni—P plated aluminum alloy substrate and a glass substrate made of crystallized glass or reinforced glass are more preferable, and an Ni—P plated aluminum alloy substrate is even more preferable.

In addition, the polishing composition of the present invention is suitably used for those made of at least silicon on the side of the substrate to be polished. For example, a glass substrate made of crystallized glass or reinforced glass or a semiconductor substrate in which a thin film made of silicon is formed on the surface of the substrate, even more preferably a glass substrate made of crystallized glass or reinforced glass. Therefore, the present invention relates to a method of polishing a glass substrate with the polishing composition of the present invention.

The shape of the substrate to be polished is not particularly limited. For example, those having shapes containing planar portions such as discs, plates, slabs and prisms, or shapes containing curved portions such as lenses can be subjects for polishing with the polishing composition of the present invention. Among them, those having disc-shaped substrates are even more preferable in polishing.

The polishing composition of the present invention can be preferably used in polishing a substrate for precision parts. For example, the polishing composition is suitable for polishing substrates for magnetic recording media such as magnetic disks including memory hard disks, optical disks, and opto-magnetic disks; and precision parts such as photomask substrates, optical lenses, optical mirrors, optical prisms and semiconductor substrates, and the like. Among them, since the polishing composition of the present invention can remarkably reduce nano scratches important in high density or high integration, the polishing composition is more preferable for polishing a magnetic disk substrate such as a memory hard disk substrate, or a semiconductor substrate, even more preferable for polishing a memory hard disk substrate. As the memory hard disk substrate, a glass memory hard disk substrate or Ni—P plated substrate is more preferable. Therefore, the present invention relates to a method of polishing a memory hard disk substrate with the polishing composition of the present invention.

The polishing of a memory hard disk substrate or a semiconductor substrate includes, for example, the steps of polishing a silicon wafer (bare wafer), forming a film for shallow trench isolation, subjecting an interlayer dielectric to planarization, forming an embedded metal line, and forming an embedded capacitor, and the like.

The surface properties of the substrate before subjecting to the polishing process with the polishing composition of the present invention are not particularly limited. For example, those substrates having surface properties that Ra is 1 nm are preferable.

The polishing composition of the present invention is especially effective in the polishing step, and the polishing composition can be similarly applied to polishing steps other than these, for example, lapping step, and the like.

2. Method for Manufacturing Substrate

The present invention relates to a method for manufacturing a substrate.

One of the features of the method for manufacturing a substrate of the present invention resides in that the method includes the step of polishing a substrate to be polished with a polishing composition containing an aqueous medium and silica particles, wherein the silica particles in the polishing composition has a zeta potential of from −15 to 40 mV. By having the feature, there are exhibited some effects that the polished object has a small surface roughness with a high polishing rate, and that nano scratches can be remarkably reduced.

In the method for manufacturing a substrate of the present invention, the above-mentioned polishing composition of the present invention is suitably used.

The silica particles used in the method for manufacturing a substrate of the present invention may be the same ones as those used in the above-mentioned polishing composition of the present invention.

Among them, the silica particles having an average primary particle size of 1 nm or more are preferable, more preferably 3 nm or more, even more preferably 5 nm or more, from the viewpoint of an increase in the polishing rate. Also, the silica particles having an average primary particle size of less than 40 nm are preferable, more preferably 35 nm or less, even more preferably 30 nm or less, even more preferably 25 nm or less, even more preferably 20 nm or less, from the viewpoint of reducing surface roughness. Therefore, the silica particles have an average primary particle size of preferably 1 nm or more and less than 40 nm, more preferably from 1 to 35 nm, even more preferably from 3 to 30 nm, even more preferably from 5 to 25 nm, even more preferably from 5 to 20 nm, from the viewpoint of economically reducing surface roughness. Further, when the primary particles are aggregated to form secondary particles, the silica particles have a secondary average particle size of preferably from 5 to 150 nm, more preferably from 5 to 100 nm, even more preferably from 5 to 80 nm, even more preferably from 5 to 50 nm, even more preferably from 5 to 30 nm, from the viewpoint of an increase in the polishing rate and from the viewpoint of reduction of surface roughness of a substrate in the same manner as above.

The polishing step used in the method for manufacturing a substrate of the present invention may be the same ones as the polishing step used in the above-mentioned polishing composition of the present invention. The polishing step may be preferably carried out in a second or subsequent step among the plural polishing steps, and it is even more preferable to carry out the polishing step as a final polishing step. In this polishing step, in order to avoid admixing of the abrasive of the previous step or the polishing composition, separate polishing machines may be used. And when the separate polishing machines are used, it is preferable to clean the substrate for each step. Here, the polishing machines are not particularly limited.

b 3. Method for Increasing Polishing Rate of Substrate to be Polished

In addition, the present invention relates to a method for increasing a polishing rate of a substrate to be polished (hereinafter referred to as “polishing rate-increasing method”).

One of the features of the polishing rate-increasing method of the present invention resides in that the method includes the step of adjusting a zeta potential of silica particles in a polishing composition containing an aqueous medium and silica particles to −15 to 40 mV. By having the above feature, the polishing rate can be increased while also satisfying surface smoothness.

The aqueous medium and the silica particles used in the polishing rate-increasing method of the present invention may be the same ones as those used in the above-mentioned polishing composition of the present invention.

Therefore, the polishing composition of the present invention can be suitably used for the polishing rate-increasing method of the present invention.

Also, the polishing steps may be the same as those polishing steps used in the polishing composition of the present invention as mentioned above.

The polishing rate-increasing method of the present invention can be preferably used in polishing a substrate for precision parts. For example, the polishing composition is suitable for polishing substrates for magnetic recording media, such as magnetic disks, optical disks, opto-magnetic disks such as memory hard disk substrates, and substrates for precision parts such as photomask substrates, optical lenses, optical mirrors, optical prisms and semiconductor substrates, and the like. The polishing of a semiconductor substrate includes, for example, the steps of polishing a silicon wafer (bare wafer), forming a film for shallow trench isolation, subjecting an interlayer dielectric to planarization, forming an embedded metal line, and forming an embedded capacitor, and the like.

Also, another embodiment of the method for manufacturing a substrate of the present invention includes a method for manufacturing a substrate including the step of applying the above-mentioned polishing rate-increasing method of the present invention to a substrate to be polished. Specifically, one of the features of this embodiment of the method for manufacturing a substrate resides in that the method includes the step of applying a method for increasing a polishing rate of a substrate to be polished with a polishing composition containing an aqueous medium and silica particles, including the step of adjusting a zeta potential of silica particles in the polishing composition to −15 to 40 mV, to a substrate to be polished. By having the feature, there are exhibited some effects that the polishing rate can be increased while keeping a low scratching property owned by the silica particles, and that the production efficiency can be enhanced.

Since the method has the above feature, the method can be applied to the manufacture of substrates for magnetic disks such as glass memory hard disks, recording media such as optical disks and opto-magnetic disks; manufacture of semiconductor substrates such as memory ICs, logic ICs or system LSIs; and photomask substrates, optical lenses, optical mirrors, optical prisms, and the like. The method is preferably suitable for the manufacture of magnetic disks such as glass memory hard disks, or the manufacture of semiconductor substrates, more preferably for the manufacture of magnetic disks such as glass memory hard disks.

4. Method for Reducing Scratches on Substrate to be Polished

In addition, the present invention relates to a method for reducing scratches on a substrate to be polished with the polishing composition (hereinafter simply referred to as “scratch-reducing method”).

One of the features of the scratch-reducing method of the present invention resides in that the method for reducing scratches on a substrate to be polished with a polishing composition containing an aqueous medium and silica particles, including the step of adjusting a zeta potential of silica particles in the polishing composition to −15 to 40 mV. By having the feature, the scratches on the substrate to be polished can be reduced.

The aqueous medium and the silica particles used in the scratch-reducing method of the present invention may be the same ones as those used in the above-mentioned polishing composition of the present invention.

Therefore, the polishing composition of the present invention can be suitably used for the scratch-reducing method of the present invention.

Also, the polishing steps may be the same as those polishing steps used in the polishing composition of the present invention as mentioned above.

5. Manufactured Substrate

The substrate manufactured by using the polishing composition of the present invention or using the method for manufacturing a substrate of the present invention as described above has excellent surface smoothness. For example, those substrates having surface roughness (Ra) of 0.3 nm or less, preferably 0.2 nm or less, more preferably 0.15 nm or less, even more preferably 0.13 nm or less are obtained.

Also, the manufactured substrate has very little nano scratches. Therefore, when the substrate is, for example, a memory hard disk substrate, the substrate can meet the requirement of a recording density of preferably 120 G/inch², and more preferably 160 G/inch², and when the substrate is a semiconductor substrate, the substrate can meet the requirement of a wire width of preferably 65 nm, and more preferably 45 nm.

EXAMPLES

The following examples further describe and demonstrate embodiments of the present invention. The examples are given solely for the purposes of illustration and are not to be construed as limitations of the present invention.

Each of the polishing compositions in the following Examples and Comparative Examples was evaluated for its polishing properties by using an Ni—P plated, aluminum alloy substrate having a thickness of 1.27 mm, an outer circumferential diameter of 95 mm and an inner circumferential diameter of 25 mm, which was previously roughly polished with a polishing liquid containing alumina abrasives so that the substrate had a surface roughness (Ra) of 1 nm as an object to be polished.

Examples I-1 to I-9 and Comparative Examples I-1 to I-5

There were added together Colloidal Silica I-A (commercially available from Du Pont, average primary particle size: 27 nm, D90/D50=3.1), Colloidal Silica I-B (commercially available from Du Pont, average primary particle size: 15 nm, D90/D50=2.2), Colloidal Silica I-C (commercially available from Du Pont, average primary particle size: 19 nm, D90/D50=1.6), or a mixture of Colloidal Silicas I-A and I-B corresponding to Example I-4 (commercially available from Du Pont, average primary particle size: 18 nm, D90/D50=3.0) as an abrasive; a 60% by weight aqueous HEDP solution, a 98% by weight sulfuric acid, and/or citric acid as a zeta potential controlling agent; and a 35% by weight aqueous hydrogen peroxide as the other component, to give each of the polishing compositions having a composition, pH, and a zeta potential of the abrasive as shown in Table 1. Here, the balance was ion-exchanged water.

The order of mixing each component was as follows: The aqueous hydrogen peroxide was added to an aqueous solution prepared by diluting a zeta potential controlling agent HEDP, sulfuric acid or citric acid in water, thereafter the remaining components were added, mixed and adjusted. The resulting mixture was added little by little to the colloidal silica slurry while stirring, to give a polishing composition.

Zeta potential, nano scratches, and surface roughness (Ra) of each of the polishing compositions obtained in Examples I-1 to I-9 and Comparative Examples I-1 to I-5 were determined and evaluated in accordance with the following methods. The results are shown in Table 1.

I-1. Polishing Conditions

-   Polishing testing machine: double-sided processing machine, Model     9B, commercially available from SPEEDFAM CO., LTD. -   Polishing cloth (pad): a cloth for finish-polishing commercially     available from FUJIBO (thickness: 0.9 mm, an open pore diameter: 30     μm, Shore A hardness: 60°) -   Rotational speed of the platen: 32.5 r/min -   Flow rate for the polishing composition: 100 mL/min -   Polishing time: 4 minutes -   Polishing load: 7.8 kPa -   Number of substrates introduced: 10     I-2. Determination Conditions for Zeta Potential -   Determination device: “ELS-8000” commercially available from Otsuka     Electronics Co., Ltd. (flat plate cell type) -   Applied voltage: 80V -   Determination temperature: 25° C. -   Determination sample: A polishing composition prepared by diluting     an abrasive with an aqueous solution of a zeta potential controlling     agent (corresponding to an aqueous solution containing a zeta     potential adjusting agent and water) of which the pH was adjusted to     be the same one as that of the polishing composition, so as to have     an abrasive concentration of 0.05% by weight as a determination     sample. -   Number of determinations: Determinations were made three times using     the same sample under the same determination conditions, and an     average of the three determinations was defined as the zeta     potential.     I-3. Determination Conditions for Nano Scratches -   Determination device: “Micromax VMX-2100CSP” (commercially available     from VISION PSYTEC CO., LTD.) -   Light source: 2Sλ (250 W) and 3Pλ (250 W), both being 100% -   Tilted angle: −6° -   Magnification: Maximum (vision scope: 1/120 of the entire area -   Observation scope: Entire area (substrate having an outer     circumferential diameter of 95 mm and an inner circumferential     diameter of 25 mm) -   Iris: notch -   Evaluation: Four substrates are selected at random from the     substrates introduced into a polishing test machine. The number of     nano scratches (without unit, -) per one side of the substrate was     calculated by dividing the total of the number of nano scratches on     each of both sides of the four substrates by a factor of 8. Also,     the nano scratches shown in the table were evaluated relative to the     number of nano scratches (/side) of Comparative Example 1.     I-4. Determination Conditions for Surface Roughness (Ra) -   Determination device: “Nano Scope III, Dimension 3000” commercially     available from Digital Instrument -   Scan rate: 1.0 Hz -   Scan area: 2×2 μm

Evaluation: Determinations were made at three points at an equidistance from the inner circumference and the outer circumference in an interval of 120°, and the determinations were made on both sides of the substrate. An average of a total of 6 points was obtained. TABLE I Composition of Polishing Composition (% by weight)¹⁾ Zeta Potential Other Abrasive Controlling Agent Components Zeta Polishing Colloidal Colloidal Colloidal Sulfuric Citric Hydrogen Potential Nano Ra Rate Silica I-A Silica I-B Silica I-C HEDP Acid Acid Peroxide (mV) pH Scratches (nm) (μm/min) Ex. I-1 7 — — 5.6 — — — 5 1 0.18 0.16 0.09 Ex. I-2 7 — — 0.13 0.55 — — 1 1.2 0.13 0.16 0.10 Ex. I-3 — 7 — 0.28 — — — −0.1 3 0.16 0.13 0.05 Ex. I-4 3.5 3.5 — 2 — — 0.6 −0.1 1.8 0.04 0.12 0.16 Ex. I-5 7 — — 2 — — — −0.2 1.8 0.13 0.18 0.08 Ex. I-6 7 — — 0.24 — — — −8 3 0.22 0.19 0.06 Ex. I-7 7 — — — — 0.67 — −10 3 0.33 0.19 0.06 Ex. I-8 — — 7 0.3 — — — −15 3 0.56 0.21 0.05 Ex. I-9 — 7 — 5.5 — — — 25 1.5 0.40 0.16 0.06 Comp. 7 — — 0.12 — — — −73 7 1.00 0.38 0.01 Ex. I-1 Comp. 7 — — 0.16 — — — −40 5 0.82 0.34 0.02 Ex. I-2 Comp. — 7 — 0.15 — — — −63 7 0.85 0.31 0.01 Ex. I-3 Comp. — — 7 0.16 — — — −72 7 1.31 0.29 0.01 Ex. I-4 Comp. — 7 — 6.5 — — — 35 1.0 0.70 0.19 0.07 Ex. I-5 Note ¹⁾: The balance of the polishing composition is ion-exchanged water.

It can be seen from the results shown in Table 1 that the substrate obtained by using the polishing compositions of Examples I-1 to I-9 suppressed the generation of nano scratches and reduced surface roughness, as compared to those of Comparative Example I-1 to I-5.

(Determination Conditions for Zeta Potential)

The determination conditions for the zeta potential given hereinbelow are as follows.

-   Determination device: “NICOMP Model-380 ZLS” (commercially available     from Particle Sizing Systems) -   Applied voltage: 1.0 to 5.0 V/cm -   Determination sample: Each of the polishing compositions obtained in     Examples and Comparative Examples was separated by a centrifuge     (centrifugal force: 35000 g, 30 minutes), and the supernatant was     collected. The polishing composition was added in an amount of 0.2%     by weight to the supernatant while mixing, to be used as the     determination sample. -   Number of determinations: Determinations were made three times using     the same sample under the same determination conditions, and an     average of the three determinations was defined as the zeta     potential.

Example II-1

There were added together 20% by weight of Colloidal Silica Slurry II-A (commercially available from Du Pont, average primary particle size: 37 nm, D90/D50=2.2) as silica particles; 0.25% by weight of a 36% by weight aqueous hydrochloric acid solution as a zeta potential controlling agent, and the balance being ion-exchanged water to give a polishing composition (zeta potential: 26.5 mV, pH: 1.5).

The order of mixing each component was as follows: The zeta potential controlling agent 36% by weight aqueous hydrochloric acid solution prepared by diluting hydrochloric acid with water was added to Colloidal Silica Slurry II-A little by little while stirring, to give a polishing composition. The polishing properties were evaluated on the basis of the following conditions by using the polishing composition. As a result, the polishing rate was 0.197 μm/minute, and a surface smoothness (Ra) of 0.23 nm.

II-1. Substrate to be Polished

A memory hard disk substrate made of crystallized glass, an outer circumference of 65 mm, an inner circumference of 20 mm, a thickness of 0.65 mm and surface roughness (Ra) of 0.2 to 0.3 nm

II-2. Polishing Conditions

-   Polishing device: “Musasino Denshi MA-300,” (a single-sided     polishing machine, platen diameter: 300 mm, carrier forced driving     type) -   Rotational speed of platen: 90 r/min -   Rotational speed of carrier: 90 r/min -   Flow rate for the polishing composition: 50 mL/min (1.7 mL/min per 1     cm² of the substrate to be polished) -   Polishing time: 10 minutes -   Polishing load: 14.7 kPa -   Polishing pad: “suede type, Bellatrix N0012” (commercially available     from Kanebo, LTD.) -   Dressing method: Brush-dressing was carried out for 30 seconds for     every polishing.     II-3. Calculation Method for Polishing Rate

Supposing that the specific gravity of the substrate to be polished was 2.41, the polishing rate (μm/minute) was calculated from the amount of weight loss before and after the polishing.

[Method for Evaluating Surface Smoothness of Substrate]

The surface smoothness of the substrate was evaluated by determining an average surface roughness (Ra) of the substrate. The conditions were as follows.

-   -   Device: Zygo New View 5032     -   Lens: Magnification, 10 times     -   Zooming Ratio: 1     -   Camera: 320×240 Normal     -   Remove: Cylinder     -   Filter: FFT Fixed Band Pass     -   0.005 to 0.1 mm     -   Area: 0.85 mm×0.64 mm

Examples II-2 to II-4, Comparative Example II-1

Each of the polishing compositions having a composition, a pH and a zeta potential of the silica particles as shown in Table 2 was prepared in the same manner as in Example II-1, and the polishing properties were evaluated. The results for the polishing rate and the zeta potential are shown in Table 2. TABLE 2 Composition of Polishing Composition (% be weight) Silica Particles Zeta Potential Zeta Polishing Colloidal Silica Controlling Agent Potential Rate Slurry II-A Hydrochloric Acid (mV) pH (μm/min.) Ex. II-2 20 0.11 −0.7 4.0 0.154 Ex. II-3 20 0.09 −5.7 6.4 0.119 Ex. II-4 20 0.07 −9.0 8.0 0.100 Comp 20 0 −17.9 10.5 0.059 Ex. II-1

It can be seen from the results of Table 2 that the polishing compositions obtained in Examples II-2 to II-4 in which the zeta potential of the silica particles in the polishing composition was adjusted within the range from −15 to 40 mV showed remarkable increase in the polishing rates, as compared to that of Comparative Example II-1.

Examples II-5 to II-6, Comparative Examples II-2

There were added together Colloidal Silica Slurry II-B (commercially available from Du Pont, average primary particle size: 17 nm, D90/D50=1.6) as silica particles, and a 36% by weight aqueous hydrochloric acid solution as a zeta potential controlling agent in amounts shown in Table 3, to give a polishing composition having a composition, a pH and a zeta potential of the silica particles as shown in Table 3. Here, the balance was ion-exchanged water.

The order of mixing each component was as follows: The zeta potential controlling agent aqueous hydrochloric acid solution prepared by diluting hydrochloric acid with water was added to Colloidal Silica Slurry II-B little by little while stirring, to give a polishing composition. The polishing properties were evaluated on the basis of the following conditions by using the polishing composition. The results for the polishing rate and the zeta potential are shown in Table 3. The substrate to be polished, the polishing conditions and the calculation method for the polishing rate are the same as those of Examples II-1 to II-4. TABLE 3 Composition of Polishing Composition (% be weight) Silica Particles Zeta Potential Zeta Polishing Colloidal Controlling Agent Potential Rate Slurry II-B Hydrochloric Acid (mV) pH (μm/min) Ex. II-5 20 0.27 24.1 1.5 0.193 Ex. II-6 20 0.13 0.1 4.0 0.145 Comp. 20 0 −17.2 10.5 0.058 Ex. II-2

It can be seen from the results of Table 3 that the polishing compositions obtained in Examples II-5 and II-6 in which the zeta potential of the silica particles in the polishing composition was adjusted within the range from −15 to 40 mV showed remarkable increase in the polishing rates, as compared to that of Comparative Example II-2.

Examples II-7 to II-8, Comparative Examples II-3

There were added together Colloidal Silica Slurry II-A as silica particles, and a 36% by weight aqueous hydrochloric acid solution as a zeta potential controlling agent in amounts shown in Table 4, to give a polishing composition having a composition, a pH and a zeta potential of the silica particles as shown in Table 4. Here, the balance was ion-exchanged water. The order of mixing each component was as follows: The zeta potential controlling agent aqueous hydrochloric acid solution prepared by diluting hydrochloric acid with water was added to Colloidal Silica Slurry II-A little by little while stirring, to give a polishing composition. The polishing properties were evaluated on the basis of the following conditions by using the polishing composition. The results for the polishing rate and the zeta potential are shown in Table 4. The substrate to be polished, the polishing conditions and the calculation method for polishing rate are the same as those of Examples II-1 to II-4 except that a substrate made of reinforced glass was used for the substrate to be polished. TABLE 4 Composition of Polishing Composition (% by weight) Silica Particles Zeta Potential Zeta Polishing Colloidal Silica Controlling Agent Potential Rate Slurry II-A Hydrochloric Acid (mV) pH (μm/min) Ex. II-7 20 0.25 26.5 1.5 0.538 Ex. II-8 20 0.09 −5.7 6.4 0.305 Comp 20 0 −17.9 10.5 0.177 Ex. II-3

It can be seen from the results of Table 4 that the polishing compositions obtained in Examples II-7 and II-8 in which the zeta potential of the silica particles in the polishing composition was adjusted within the range from −15 to 40 mV showed remarkable increase in the polishing rates, as compared to that of Comparative Example II-3.

Example II-9 to II-10, Comparative Examples II-4

There were added together Colloidal Silica Slurry II-A as silica particles, and a 36% by weight aqueous hydrochloric acid solution as a zeta potential controlling agent in amounts shown in Table 5, to give a polishing composition having a composition, a pH and a zeta potential of the silica particles as shown in Table 5. Here, the balance was ion-exchanged water. The order of mixing each component was as follows: The zeta potential controlling agent aqueous hydrochloric acid solution prepared by diluting hydrochloric acid with eater was added to Colloidal Silica Slurry II-A little by little while stirring, to give a polishing composition. The polishing properties were evaluated on the basis of the following conditions by using the polishing composition. The results for the polishing rate and the zeta potential are shown in Table 5.

II-4. Substrate to be Polished

PE-TEOS film having a thickness of 2000 nm was formed on an 8-inch (200 mm) silicon substrate, and the film-forming substrate was cut into squares of 40 mm×40 mm.

II-5. Polishing Conditions

The polishing conditions were the same as those of Examples II-1 to II-4 except that the flow rate for the polishing composition, the polishing time, the polishing pad and the dressing method were as follows.

-   Flow rate for the polishing composition: 200 mL/min (0.6 mL/min per     1 cm² of the substrate to be polished) -   Polishing time: 5 minutes -   Polishing pad: “IC1000 050(P)/Suba400” (commercially available from     RODEL NITTA) -   Dressing method: Dressing was carried out with “Diamond Dresser     #100” for 30 seconds for every polishing.     II-6. Calculation Method for Polishing Rage

The polishing rate (nm/min) was determined from the difference between the thickness of the remaining PE-TEOS film before polishing and that of the remaining film after polishing. The thickness of the remaining film was determined using a light interference-type film thickness gauge (LAMBDA ACE VM-1000, commercially available from DAINIPPON SCREEN MFG. CO., LTD.). TABLE 5 Composition of Polishing Composition (% by weight) Silica Particles Zeta Potential Zeta Polishing Colloidal Silica Controlling Agent Potential Rate Slurry II-A Hydrochloric Acid (mV) pH (μm/min) Ex. II-9 20 0.25 26.5 1.5 0.184 Ex. II-10 20 0.09 −5.7 6.4 0.143 Comp 20 0 −17.9 10.5 0.139 Ex. II-4

It can be seen from the results of Table 5 that the polishing compositions obtained in Examples II-9 and II-10 in which the zeta potential of the silica particles in the polishing composition was adjusted within the range from −15 to 40 mV showed remarkable increase in the polishing rates, as compared to that of Comparative Example II-4.

The polishing composition of the present invention can be favorably used in polishing the substrate for precision parts, including substrates for magnetic recording media such as magnetic disks, optical disks and opto-magnetic disks; photomask substrates; optical lenses; optical mirrors; optical prisms; semiconductor substrates; and the like.

The present invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims. 

1. A method for reducing scratches on a substrate to be polished with a polishing composition comprising an aqueous medium and silica particles, comprising the step of adjusting a zeta potential of silica particles in the polishing composition to −15 to 40 mV.
 2. The method according to claim 1, wherein the silica particles have an average primary particle size of 1 nm or more and less than 40 nm.
 3. The method according to claim 1, wherein the substrate to be polished is a memory hard disk substrate.
 4. The method according to claim 1, further comprising the step of pressing a polishing pad against the substrate to be polished while feeding the polishing composition at a rate of from 0.01 to 3 mL/minute per 1 cm² of the substrate to be polished. 